1. Field of the Invention
The present invention generally relates to a plating technique, and more particularly, to a whiskerless and lead-free plated structure and plating method involving the formation of a tin-based plating coating used in place of a tin-lead (Sn—Pb) plating film.
2. Description of Related Art
Conventionally, tin-lead (Sn—Pb) solder plating is applied to metallic devices and components, such as connector terminals or lead frames for semiconductor integrated circuits. In recent years and continuing, however, lead-free plating has been discussed and studies are being conducted to perform lead-free plating in place of conventional tin-lead solder plating from the viewpoint of protection of the environment. Candidates for lead-free plating include leadless tin plating, tin-copper (Sn—Cu) alloy plating, tin-bismuth (Sn—Bi) alloy plating, and tin-silver (Sn—Ag) alloy plating. Japanese Patent Laid-Open Application Publication No. 2001-26898 discloses a tin-copper alloy plating method for forming a tin-copper alloy plating film containing 0.01 to 10.0 weight percent copper.
When a tin-based lead-free alloy (including the above-listed alloys) is used for a plating film, tin crystal whiskers are likely to grow depending on the types of plating alloys. Tin whiskers often cause electric failure or short-circuiting between adjacent electrodes. Because whiskers are as thin as 1 μm diameter and as long as 1000 μm or more, they are easily separated from the plating film. The dispersed whiskers cause further problems, including short-circuiting, in and outside devices.
One reason for the generation of whiskers is internal stress in a plating film. It is considered that whiskers grow during re-crystallization of tin under the application of internal stress.
This is explained in more detail with reference to FIG. 1. If a tin (Sn) or tin-alloy plating film 101 is applied onto a copper (Cu) or copper-alloy underlayer 100 (which underlayer is, for example, a lead frame or a plating film of the lead frame), then Cu6Sn5 intermetallic compound 102 is produced at the interface between the copper film 100 and the tin (or tin alloy) plating film 101. This Cu6Sn5 compound 102 grows greatly at the grain boundary and applies compression stress to the Sn plating film 101, which causes internal stress to be generated in the Sn plating film 101.
It is known that Cu6Sn5 expands cubically because its density is 8.3, which value is relatively small. Copper atoms constituting the underlayer Cu film 100 diffuse rapidly at the grain boundary of the tin (Sn) plating film 101, and cubically expansive Cu6Sn5 compound 102 is produced at the tin plating grain boundary. For this reason, the tin plating film 101 is subjected to compression stress. This compression stress applied to the inside of the tin plating film 101 acts as a driving force for generation of crystal whiskers, and consequently, long and needle-like whiskers are produced.
In general, whiskers may be prevented from growing by performing mat electrodeposition or semibright plating using plating solution with a greatly reduced amount of brightener agent to reduce the internal stress. Whiskers may also be prevented by applying a thermal process of about 150° C. after plating to reduce the stress.
In addition, it is proposed to form a diffusion barrier film from nickel or other suitable metals on the Cu or Cu-alloy underlayer (or base) to prevent growth of Cu6Sn5 and generation of whiskers. See, for example, Kensaku MOURI, Koutaro TAKEUCHI, and Yoshinori OMOTO, “Development of Lead-Free Solder Plating”, Kouchi-Ken Kogyo Gijutsu Center Research Report No. 34, 2003.
However, the method of forming a diffusion barrier film causes another problem because a nickel plating film easily diffuses into a tin plating film. Due to this property, nickel separates out through the grain boundary and precipitates at the surface of the plating film. The nickel precipitate forms nickel oxide at the plating surface, which oxide greatly reduces solder wetness.
The former method for reducing internal stress by use of mat electrodeposition or semibright plating, or by application of a post-plating thermal process may reduce whiskers to a certain extent; however, the whisker prevention effect is insufficient.